After an oil or gas well has been drilled to a specified depth, the next procedure typically involves placing casing in the well to some depth. The casing is typically cemented in place to prevent external leakage along the well borehole. One of the important aspects in the well completion procedure involves the use of explosives including perforating gun assemblies. These gun assemblies are used to perforate holes in the cemented casing to allow the production of downhole hydrocarbons. A typical gun assembly can range from a few feet to several hundred feet in length and typically is deployed in the well borehole supported on a tubing or wireline string. This includes one or more perforating guns, and typically hundreds of shaped explosive charges deployed along the length of the perforating gun assembly. These devices are formed of high explosives which are interconnected by a detonating cord. If prematurely detonated, they can cause severe damage to the equipment including the partially completed well. If prematurely detonated at or near the surface, they can injure and even kill surface located personnel. With a view towards safety, the use of explosive detonators is especially dangerous, even more so in the crowded confines of an offshore drilling platform. The risk is enhanced or increased by the presence of radio frequency energy as well as AC power generators. Practically every offshore drilling platform includes operative radio frequency signal generating devices and also AC power generators. In light of this hostile environment in close quarters with a number of personnel in the immediate vicinity of the perforating gun assembly when it is assembled at the well surface, it is extremely important to use detonators which are very difficult to fire or set off, and which might set off the explosive string including the detonating cord and numerous shaped charges.
The term secondary explosive, in contrast with the term primary explosive, defines an energetic material which is relatively insensitive to initiation by external stimuli, such as heat, impact friction, and static discharge. The term exploding foil initiator (EFI) describes a type of detonator that utilizes only secondary explosives. EFI's require very high power inputs (megawatts) to function and are considered extremely safe. They are well known in the explosive art.
The use of a relatively insensitive secondary explosive in an exploding foil initiator (EFI) renders such a device rather insensitive to accidental initiation. An EFI using the popular secondary explosive HNS-4 requires a high current DC pulse. The pulse can be on the order of 2500-3000 volts to reliably trigger the device. Such a device used in conjunction with a perforating gun assembly provides a thermally stable secondary explosive mechanism which renders the EFI safe for use in a perforating gun assembly, and is therefore quite safe because of its lack of sensitivity. By contrast however, this requires a larger voltage pulse, and a larger current in the pulse to initiate detonation. The generation of such a high voltage, high current pulse at great depths in a well borehole requires a downhole firing unit (consisting of safe/arm circuits, power supplies, and firing capacitors) which is deployed on the logging cable at the perforating gun assembly. This unit must be capable of operating at the typical elevated pressures and temperatures which are encountered in the well borehole and must be of sufficiently small size to pass through downhole tubulars and restrictions. The EFI of the present disclosure is a device which is ideally installed in the perforating gun assembly so that it can be lowered into a well borehole. It is an exploding foil assembly which has the inherent safety features appropriate to prevent premature detonation. In conjunction with a firing unit installed in the perforating gun assembly, the present disclosure sets forth an exploding foil initiator which actually fires at a reduced voltage which enables use of a smaller and more compact firing unit. This disclosure sets forth such an exploding foil system which has a lower voltage requirement and yet which accomplishes firing without undue sacrifice in safety.
One method of reducing the firing voltage of such a system is to utilize a secondary explosive which is more susceptible to initiation by a short duration shock pulse in comparison with those required of other secondary explosives. This highly desirable explosive material is a benzene ring explosive, sometimes known as BRX. This explosive material will initiate at a lower voltage, around 2000 volts. That is substantially less than the voltage required for the explosive HNS-4. Yet, even though it does detonate at 2000 volts, it will not initiate at levels around 1100 volts. This defines a very safe device because stray voltages at these levels are not usually present during normal oil rig operations. In addition, the thermal stability of BRX exceeds that of HNS, thus reinforcing its desirability for downhole use.